While the actual CSS used will depend on the active template for the site, but there should be a checkout_one.css file present in includes/templates/royal_fleur/css.

To the bottom of that stylesheet, you can add:
Code:
#opc-order-confirm btn {
    padding: .5rem 1rem;
    font-size: 1.25rem;
    line-height: 1.5;
    border-radius: .3rem;
}
That, essentially, emulates Bootstrap's btn-lg class. See the w3schools docs for additional information: https://www.w3schools.com/bootstrap4...ap_buttons.asp